In the electrostatic coating of workpieces, which are usually placed by an automatic conveyor system in a spray-cabin, or the like, coating station and are moved relatively to one or more paint-spraying devices, a high voltage is applied between the workpiece and each paint-spraying device. In order to take account of the operating conditions (the workpiece, the type of enamel, atmospheric conditions, etc.), the magnitude of the voltage must be variable, i.e. between about 60 and 110 V in the case of vehicle bodies.
During the coating operation, care must be taken for safety reasons to ensure that the strength of the electrical field cannot reach a value which could produce a dangerous voltage breakdown or spark discharge. A disruptive field strength occurs when the distance between the paint-spraying device and the workpiece is insufficient, i.e. if the guidance of the workpiece is unstable or in the case of vehicle bodies, if doors or hoods are inadvertently opened. Since there is no practical way of measuring the strength of the field in a coating installation, the operating current of the high-voltage source is monitored. The installation is automatically shut off when the operating current reaches a predetermined current threshold which, if is exceeded, presents a danger of voltage breakdown. Since the current flowing between the high-voltage electrodes rises sharply just before reaching the disruptive field strength, the danger of a voltage breakdown can be eliminated as long as the operating current does not exceed the normal value, which is measured at the beginning of the operation, by more than a predetermined amount.
However, accurate and reliable adjustment of the relevant current threshold presents considerable problems. The operating current, especially in the upper range of selectable voltage values, does not change linearly with the voltage. The measured operating current is only partly the current-flowing between the electrodes and has additional components, including a shunt-current flowing from the high-voltage source through the enamel-supply system. At the start of the operation, this shunt-current may already be substantially higher than the electrode-current and as the operation proceeds, it may gradually increase considerably as a result of increasing contamination of its path. The magnitude of this current, however, is no indication of the field strength to be monitored. On the other hand, it is not immediately possible to measure the electrode-current only.
According to a method now commonly used, in the case of the electrostatic coating installation for vehicle bodies in which the high voltage can be varied by means of a step-switch, for example 5 steps of 10 kV each, the normal operating current for the voltage step selected is measured before the coating operation starts. A current threshold is adjustable manually by means of a potentiometer associated with the voltage step selected. The current threshold is higher by a specific amount than the measured normal current. If the voltage is altered, the procedure is repeated with another potentiometer associated with the new voltage step.
For various reasons this method is not satisfactory. In the first place, it would be desirable to be able to vary the voltage in substantially smaller steps, or quasi steplessly, but this cannot be done with the conventional method since the number of potentiometers would have to equal the number of selectable voltage-steps. This would not only be costly, but would also take up too much space. It must also be taken into account that it is usual to have several simultaneously operating sprayers in a coating station and that the high-voltage field of each sprayer requires its own monitoring system. For instance, if there are 10 sprayers, and if their respective operating voltages are to be variable between 60 and 110 kV in one 1 V steps, a total of 500 potentiometers would have to be installed.
Another disadvantage is the effort and possible unreliability associated with manual adjustment of the potentiometers which determine the current threshold. The fact is that these potentiometers must be located in locked control-boxes or must be protected in some other way from unauthorized adjustment. Finally, unwanted shutdowns may occur if the current threshold is reached without any danger of voltage breakdown. This may occur as a result of increasing contamination of the aforesaid shunt-current path or because of some other relatively slow changes in operating conditions (heating-up, atmospheric changes, etc.) with the operating personnel not being informed in time.
German OS No. 27 34 341 discloses a current-monitoring system for the electrostatic coating of vehicle bodies which has steplessly selectable voltages. The safety shut-off is triggered only in the event of rapid changes in current resulting from parts of the body coming dangerously close to the high-voltage spraying units. It is not triggered by gradual changes in current caused, for example, by gradual contamination of the installation. The operating current is measured at frequent intervals (every 200 ms) with the aid of a scanning and holding circuit. The measured value is compared with the previous measured value which has been stored as a reference value, after which the actual measured value is stored as the new reference value. Thus, the reference value with which the operating current is constantly compared grows with the gradually increasing operating current. If, within the stated time interval, the current increases by more than a specifically adjustable differential amount, the shut-off is triggered. But again, in this known system, which comprises a relatively costly circuit arrangement, dynamic current-monitoring is supplemented by a predetermined static limit-current value which must under no circumstances be exceeded. This predetermined value is set manually by the operating personnel. Here again, as in the case of the step-method described hereinbefore, there is no way of forewarning the crew. For example, if as a result of increasing contamination of the installation, the operating current approaches the static limit-value at which the unit is shut down, the crew will not be forewarned even though there is no danger of a voltage breakdown.